Cellular Elements and Efferent Fibers of the Cortex
Despite its enormous expansion throughout vertebrate evolution, the basic anatomical organization of the cerebellar cortex has remained nearly invariant. The circuitry is also among the most regular and stereotyped of any brain region. The cerebellar cortex contains eight different neuronal types: Purkinje cells, Golgi cells, granule cells, Lugaro cells, basket cells, stellate cells, unipolar brush cells, and candelabrum cells. These cells are found in all regions of the cerebellar cortex, with the exception of unipolar brush cells, which are limited mainly to cerebellar areas receiving vestibular input. These eight cell types are distributed among the three layers that make up the cerebellar cortex of higher vertebrates (
Fig. 9.21 ). The outer or superficial layer is the molecular layer; stellate and basket cells are found there. The deepest layer is the granule cell layer; this layer has the highest cellular density in the CNS and contains granule, Golgi, and unipolar brush cells. Separating the molecular and granule cell layers is the Purkinje cell layer, formed by Purkinje cell somata, which are arranged as a one-cell-thick sheet of cells. Candelabrum cells are also located in this layer. Lugaro cells are situated slightly deeper at the upper border of the granule cell layer.
The sole efferent fiber from the cortex is the Purkinje cell axon, which also has local collaterals and is GABAergic and inhibitory. Thus the remaining seven cell types are local interneurons. Of these, the stellate, basket, Golgi, Lugaro, and candelabrum cells are also inhibitory GABAergic neurons, whereas the granule and unipolar brush cells are excitatory.
Microcircuitry of the Cortex
The dendrites, axons, and patterns of synaptic connections of most neurons within the cerebellar cortex are organized with regard to the transverse (short) and longitudinal (long) axes of the folium (Fig. 9.21 ). In the vermis, where the folia run perpendicular to the sagittal plane, these axes lie in the sagittal and coronal planes, respectively. In the hemispheres, where the folia are oriented at various angles with regard to the sagittal plane, this correspondence is lost, and the local axes of the folia must then serve as the reference axes.
The Purkinje cell dendritic tree is the largest in the CNS. It extends from the Purkinje cell layer through the molecular layer to the surface of the cerebellar cortex and for several hundred microns along the transverse axis of the folium but for only 30 to 40 µm in the longitudinal direction. Thus it is like a flat pancake that lies in a plane parallel to the transverse axis of the folium. Accordingly, a set of Purkinje cell dendritic trees can be thought of as a stack of pancakes, with the stack running along the longitudinal axis of the folium.
•Fig. 9.22 Functional Connectivity of the Cerebellar Cortex. Becauseofthegeometricorganizationofthecerebellarcorticalcircuits,thefunctionalconnectivityofthecellularelementscanbedeterminedelectrophysiologically.Thefiguredepictsaclassicparadigminwhichstimulationofthecerebellarcortexactivatesabeamofparallelfibers(orange). Recordingsfromthestellateandbasketcells(green cells) andPurkinjecells(PCs;orange cells)inlinewiththisbeamshowthattheyareexcitedbytheparallelfibers.Incontrast,Purkinjecellsflankingthebeamreceiveonlyinhibition(purple areas) asaresultoftheperpendicularspatialrelationshipoftheparallelfibersandthestellateandbasketcellaxons.